I took these measurements with an OBDII scan tool starting from
cold and traveling on suburban roads for about half an hour. The
outside temperature ranged from 28 F to 36F during the trip (as
measured by the OAT, which for me continues to be correct despite
the tape on the lower grill). There was exactly 6 times that I had to
stop at lights during this trip. Mostly 40-45 mph speeds, I averaged
33 mph for the whole trip (Total Distance/Total Time).
Note that even with the tape the thermostat is almost never open!
I believe the fuel saving is accomplished by reducing the heat loss
from the engine by reducing airflow over the engine block itself!
Highway cruising (not shown) seems to keep the temperature at about
86 C, but occasionally dips below that, probably due to very low engine
load when going slightly downhill for several minutes at a time.
Probably the main fuel economy improvement due to taping is for city driving
where the highly efficient Prius engine just doesn't generate enough
waste heat to keep itself at peak temperature!
Heater was set to Fresh, Floor, 70F Auto. Nothing special was done
(except to log the OBDII data) during this trip. Sparse data near the
end of the run is the fault of the OBDII scanner software (I think the
memory leaks - it seemed to be causing the disk to swap a lot toward
the end). Disclaimer: I didn't write the OBDII software.